Buckingham Palace

Buckingham Palace, the London home of Queen Elizabeth II, is open for tour (except for the Queen's private quarters) in the summers when Her Majesty is away on vacation. When she's in town, you can book a private guided tour but it will cost you. And if you don't want to mess with admission, you can still experience Buckingham Palace by watching the Changing of the Guard, which occurs daily at 11:30 a.m., from May until late July; it occurs on alternate days the rest of the year (except during inclement weather).  

• Summer Opening. … Places are strictly limited, and it might not be possible to just turn up and get a ticket for a specific entry time, visitors should really book in advance to ensure admission." -- Wikitravel